1a) we could follow the uptake of MeHg in fish derived solely from newly deposited Hg 19. By adding a different spike Hg to the lake ( 202Hg), wetland ( 198Hg) and upland ( 200Hg) compartments of the Lake 658 watershed (52 ha in total) during a 6- to 7-year addition phase (Fig. In our experiment, these are defined as вЂambient HgвЂ™. METAALICUS addresses the relationship between changes in inorganic Hg loadings to a lake and MeHg concentrations in fish using highly enriched inorganic Hg isotopes (termed вЂspikesвЂ™) that enabled us to specifically follow a change in loading against a background of previously deposited Hg and present-day, relatively constant, Hg inputs from direct deposition to the lake surface and from the watershed. Over a 15-year period (2001вЂ“2015) we conducted a whole-ecosystem Hg loading and recovery experiment (Mercury Experiment To Assess Atmospheric Loading In Canada and the United States (METAALICUS)) in a pristine boreal watershed 19. Consequently, it is exceedingly difficult to unambiguously assess the recovery of contaminated fish populations due specifically to Hg control measures 9. In addition, until now there has been no way to evaluate the relative contribution of newly deposited Hg to contemporary MeHg production. Changing climatic conditions can also influence MeHg production 8, as well as restructure food webs, alter the dominant pathways of energy flow and cause size-dependent changes in fish growth rates that shift population size structure 14, 17, 18. Further complicating this relationship are human activities such as commercial fishing, introduction of exotic species and enhanced nutrient additions that trigger large-scale trophic disruptions 12, which can in turn substantially alter fish tissue MeHg concentrations 13, 14, 15, because fish acquire most of their MeHg through their diet 16. Yet there is little direct evidence for how quickly fish MeHg concentrations will decline following reductions in current rates of Hg loading owing, in part, to a range of ecological factors that can influence both the microbial production and the bioaccumulation of MeHg in aquatic food webs 9, 11. The Minamata Convention on Mercury is an international treaty that aims to protect human health and the environment from adverse effects of MeHg by controlling Hg emissions, which should then decrease deposition and loading of anthropogenic Hg to aquatic environments 10. Although Hg loading from watersheds may not decline in step with lowering deposition rates, this experiment clearly demonstrates that any reduction in Hg loadings to lakes, whether from direct deposition or runoff, will have immediate benefits to fish consumers. The concentration of labelled MeHg quickly decreased by up to 91% in lower trophic level organisms, initiating rapid decreases of 38вЂ“76% of MeHg concentration in large-bodied fish populations in eight years. Thereafter, isotopic additions were stopped, resulting in an approximately 100% reduction in Hg loading to the lake. The Hg isotopes became increasingly incorporated into the food web as MeHg, predominantly from additions to the lake because most of those inВ the watershed remained there. During the seven-year addition phase, we applied enriched Hg isotopes to increase local Hg wet deposition rates fivefold. Here we conducted a 15-year whole-ecosystem, single-factor experiment to determine the magnitude and timing of reductions in fish MeHg concentrations following reductions in Hg additions to a boreal lake and its watershed. Predicting the efficacy of Hg pollution controls on fish MeHg concentrations is complex because many factors influence the production and bioaccumulation of MeHg 7, 8, 9. Nature volumeВ 601, pages 74вЂ“78 ( 2022) Cite this articleДЄnthropogenic releases of mercury (Hg) 1, 2, 3 are a human health issue 4 because the potent toxicant methylmercury (MeHg), formed primarily by microbial methylation of inorganic Hg in aquatic ecosystems, bioaccumulates to high concentrations in fish consumed by humans 5, 6. Experimental evidence for recovery of mercury-contaminated fish populations

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